EUVD-2026-36054Severity by source
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Primary rating from Vendor (EEF) · only source for this CVE.
CVSS VectorVendor: EEF
Lifecycle Timeline
3DescriptionCVE.org
Observable Timing Discrepancy vulnerability in Erlang/OTP ssh (ssh_auth, ssh_options modules) allows unauthenticated remote username enumeration via timing side-channel in password authentication.
When the SSH daemon is configured with the user_passwords or password option, ssh_auth:check_password/3 performs a PBKDF2-SHA256 computation with 600,000 iterations (~300ms) for valid usernames, but returns immediately (~0ms) for invalid usernames via the ssh_options:get_password_option/2 path. This timing difference is detectable in a single authentication attempt and allows an unauthenticated attacker to distinguish valid from invalid usernames.
The user_passwords and password options are documented as intended for test purposes; the recommended alternative is pwdfun, which is not affected by this vulnerability.
This vulnerability is associated with program files lib/ssh/src/ssh_auth.erl and lib/ssh/src/ssh_options.erl.
This issue affects OTP from OTP 29.0 before 29.0.2 corresponding to ssh from 6.0 before 6.0.1.
AnalysisAI
Username enumeration via timing side-channel in Erlang/OTP SSH daemon (OTP 29.0-29.0.1) allows unauthenticated remote attackers to distinguish valid from invalid usernames in a single probe. When the daemon is configured with the user_passwords or password options, valid usernames trigger a 600,000-iteration PBKDF2-SHA256 computation (~300ms) while invalid usernames return near-instantly (~0ms) through an early-exit path - a gap detectable without repeated attempts. No public exploit has been identified at time of analysis, and exploitation is constrained to non-default, test-oriented configurations.
Technical ContextAI
The root cause is CWE-208 (Observable Timing Discrepancy), a side-channel class where secret-dependent branching leaks information through measurable execution time. In lib/ssh/src/ssh_auth.erl, the function ssh_auth:check_password/3 invokes a 600,000-iteration PBKDF2-SHA256 derivation only for usernames matched in the user_passwords list; unrecognized usernames exit early via ssh_options:get_password_option/2 in lib/ssh/src/ssh_options.erl with no equivalent computation. CPE cpe:2.3:a:erlang:otp:*:*:*:*:*:*:*:* covers the Erlang/OTP platform's built-in SSH library. The fix at commit c342092ef4b369bb409d5b71ac8fd83bab74aedf introduces maybe_add_fake_passwd_checker/2 in ssh_options.erl, which injects a dummy PBKDF2 run for the invalid-username path when pwdfun is not configured, normalizing response timing regardless of username validity. The patched hardening documentation also now explicitly warns that custom pwdfun implementations must use constant-time comparisons to avoid reintroducing the same class of oracle.
RemediationAI
Upgrade to Erlang/OTP 29.0.2 (SSH library 6.0.1), which resolves the issue by running a dummy PBKDF2 computation on the invalid-username path, equalizing server response timing. The upstream patch is at commit c342092ef4b369bb409d5b71ac8fd83bab74aedf and the advisory is at https://github.com/erlang/otp/security/advisories/GHSA-3w6p-vwhf-wvp4. For operators who cannot upgrade immediately, replace the user_passwords or password daemon options with pwdfun, which is the documented production alternative and is not affected by this vulnerability. When implementing a pwdfun callback, use constant-time password comparison and avoid early returns based on username validity - the patched hardening guide explicitly warns that a naive pwdfun implementation can reintroduce the same timing oracle. This workaround requires code changes to the SSH daemon configuration and testing of the callback implementation for timing uniformity.
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Same weakness CWE-208 – Observable Timing Discrepancy
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